1. Field of the Invention
The present invention relates to a distance measuring equipment for obtaining a distance to an object by irradiating the object with a pulse beam, receiving the pulse beam reflected from the object and thus measuring a time spent from the irradiation and the reception thereof.
2. Related Background Art
This type of known distance measuring equipment is an optical radar system as disclosed in, e.g., Japanese Patent Laid-Open Publication No. 2-228579. FIG. 7 is a diagram illustrating a construction of a conventional distance measuring equipment. A light sending device 1 generates a pulse beam P2 by actuating a light emitting element such as a laser diode, etc. A clock generator 2, which has its output side connected to an input side of the light sending device 1, generates a clock pulse P1 serving as a generation timing for the pulse beam P2. The light sending device 1 inputs the clock pulse P1. A light receiving device 3 receives the pulse beam reflected from the object 7 irradiated with the pulse beam P2. The light receiving device 3 converts the pulse beam into an electric signal P4. A sample pulse generator 4, which has its input side connected to the other output side of the clock generator 2, counts the clock pulses P1 inputted from the clock generator 2. The sample pulse generator 4, at the same time, generates a sample pulse P3. A sample hold circuit 5 is connected to an output side of the light receiving device 3 and to an output side of the sample pulse generator 4. The sample hold circuit 5 performs sampling of the output signals P4 of the light receiving device 3 by use of the sample pulses P3 of the sample pulse generator 4. A processor 6 is connected to output sides of the sample pulse generator 4 and of the sample hold circuit 5. The processor 6 inputs a clock pulse count value from the sample pulse generator 4 and also an output signal from the sample hold circuit 5. The processor 6 thus detects a distance to the object 7.
Next, an operation of the thus constructed conventional equipment will be explained with reference to FIGS. 8 and 9. FIG. 8 is a diagram illustrating operating waveforms within a clock pulse period of the clock generator 2. FIG. 9 is a diagram illustrating operating waveforms at a time interval when this distance measuring equipment measures the distance once. The clock generator 2 generates the clock pulse P1 at a time interval T longer than a time corresponding to the maximum measured distance. This clock pulse P1 is inputted to the light sending device 1. The light sending device 1 generates the pulse beam P2 in synchronism with this clock pulse P1. The light receiving device 3 receives this pulse beam P2 reflected from the object 7. The light receiving device 3 photoelectrically converts the reflected pulse beam into the electric signal and performs a high-frequency amplification thereof. The output signal P4 therefrom is inputted to the sample hold circuit 5. On the other hand, the sample pulse generator 4 counts the clock pulses P1 given from the clock generator 2. The sample pulse generator 4 repeats counting, wherein one period is a predetermined clock pulse count value M larger than a value obtained by dividing the maximum measured distance by a distance resolving power. At the same time, the sample pulse generator 4 generates a sample pulse P3 in which the clock pulse P1 is delayed by a time given by multiplying a minute time .DELTA.T corresponding to the distance resolving power by a clock pulse count value n. The sample hold circuit 5 samples a pulse signal of the reflected beam with the above sample pulse P3. The sample hold circuit 5 holds a signal level thereof up to the next sample pulse P3. This held signal P5 is a signal in which a waveform of the high-frequency reflected pulse beam is frequency-converted into a low-frequency signal. The processor 6 compares the low-frequency output signal P5 of the sample hold circuit 5 with a threshold value Vth for detecting the reflected pulse beam and thus detects signals (A and B in FIG. 9) larger than the threshold value. A distance L between the distance measuring equipment and the object is obtained from a clock pulse count value N of the sample pulse generator 4 at this time in accordance with the following formula: EQU L=N.times..DELTA.T.times.C/2 (1)
where C is the velocity of light. Namely, the distance L is 1/2 of a pulse beam travel obtained by multiplying a light emission-through-receipt time given from the clock pulse count value N by the light velocity C. The pulse count value N is, when the count value becomes a value corresponding to the maximum detected distance, reset to 0. The above actions are defined as a measuring one period, and the distance is continuously obtained with repetitions thereof.
Problems inherent in this type of distance measuring equipment will be explained with reference to FIG. 10. A waveform shown in FIG. 10 indicates an output signal P5 of the sample hold circuit 5. The symbol LT designates a predetermined threshold value for detecting a reflected pulse beam. The processor 6 in the conventional equipment performs a detection by comparing, with the threshold value LT, a level of the light receiving signal with respect to the reflected pulse beam from the object. The processor 6 calculates a distance L by use of a count value N of the clock pulses P1 at that time. With this processing, as illustrated in FIG. 10, an error .DELTA.N is produced in the count value N of the clock pulses P1, depending on a magnitude of the level of the light receiving signal. Consequently, the measured distance L has an error given by .DELTA.N.times..DELTA.t.times.C/2. For this reason, if a reflectivity of the object 7 serving as an object for measurement is different, and even when existing at the same distance, an intensity of the reflected beam differs. For the reason given above, there arises a problem wherein a different distance L is to be measured, resulting in an error caused in the measured distance L. For example, this type of distance measuring equipment is mounted on a car. If the distance measuring equipment is utilized for a system for keeping a safe car-to-car distance by measuring a distance to a foregoing car, danger is present because the detected distance varies according to the type of the foregoing car. Namely, a measured distance error arises due to the reflectivity of the object, and becomes a serious problem which affects the safety and reliability of a system incorporating this type of equipment.